Separator for fuel cell and method of manufacture therefor

ABSTRACT

A separator ( 4 ) for a fuel cell ( 1   a ) is provided which allows effective use of heat generated in proximity to an electrolytic membrane ( 2 ) in order to melt frozen water in the gas diffusion electrodes ( 3   a,    3   b ) of the fuel cell ( 1   a ). The separator ( 4 ) is provided with a plurality of projections ( 11 ) between which a gas flow passage ( 7   a,    7   b ) on the surface of the separator ( 4 ) is formed and which make contact with the gas diffusion electrodes ( 3   a,    3   b ). The coefficient of thermal conductivity of the projection ( 11 ) is smaller than the coefficient of thermal conductivity of other sections of the separator ( 4 ).

FIELD OF THE INVENTION

This invention relates to a fuel cell and a separator used in the fuelcell and to a method of manufacture therefor.

BACKGROUND OF THE INVENTION

In a conventional polymer electrolyte fuel cell (PEFC), fuel gascontaining hydrogen and a gaseous oxidizing agent containing oxygen arerespectively supplied to two gas diffusion electrodes (cathode andanode) sandwiching a polymer electrolytic membrane. The two gasdiffusion electrodes and the polymer electrolytic membrane constitute amembrane electrode assembly (MEA). The gas diffusion electrodes aregenerally provided with a porous gas diffusion layer and a platinumcatalytic layer. The gas diffusion layer is made from carbon andsupports the catalytic layer. The platinum catalytic layer is connectedto the electrolytic membrane. Gas which is supplied from the outside ofthe diffusion layer diffuses into the platinum catalytic layer throughthe diffusion layer. Reactions as shown in Equation (1) and Equation (2)occur at the electrodes and convert chemical energy into electricalenergy.H₂→2H⁺+2e ⁻  (1)1/2O₂+2H⁺+2e ⁻→H₂O  (2)

In a PEFC, water is generated at the cathode as shown by Equation (2).Since the fuel gas must be humidified in order to facilitate the abovereactions, moisture is also supplied to the anode which performs thereactions as shown in Equation (1) above. As a result, water is alwayspresent at the gas diffusion electrode. When the fuel cell has notreached a normal operating temperature, it is sometimes the case thatwater in the gas diffusion electrode impedes the supply of gas to theplatinum catalytic layer. Therefore it is preferred that water in thegas diffusion electrode is discharged rapidly. Furthermore when theambient operating temperature of the fuel cell is low, water in the gasdiffusion electrode freezes. Thus prompt melting and discharge of waterin the gas diffusion layer electrode is preferred.

Tokkai 2000-223131 published by the Japanese Patent Office in 2000 andTokkai Hei 8-138692 published by the Japanese Patent Office in 1996disclose a fuel cell provided with a hydrophilic membrane on the surfaceforming a gas flow passage. In this fuel cell, discharge performance isimproved with respect to water produced in the passage as a result ofthe hydrophilic characteristics.

SUMMARY OF THE INVENTION

However in the prior-art fuel cell, it is difficult to ensure completedischarge of water at the gas diffusion electrode when the fuel cell isnot operating. Consequently it is difficult to avoid freezing of waterin the gas diffusion electrode at temperatures below freezing.

It is therefore an object of this invention to provide a separatordisposed between the adjacent membrane electrode assemblies in a fuelcell stack, the separator having the function of facilitating melting offrozen water (ice) between the electrolytic membrane and the separatorand the function of discharging the melted water from the gas diffusionelectrode to the gas flow groove.

It is a further object of this invention to provide a method ofmanufacturing the separator.

In order to achieve above objects, this invention provides a separatorfor a fuel cell, the fuel cell having two separators and a membraneelectrode assembly being sandwiched by the two separators, the membraneelectrode assembly being provided with an anode electrode, a cathodeelectrode and an electrolytic membrane sandwiched by the two electrodes,each of the two electrodes having a diffusion layer allowing diffusionof one of a gaseous oxidizing agent and fuel gas. The separatorcomprises a plate-like member; and a plurality of projections forforming a plurality of gas passages which allow flow of one of thegaseous oxidizing agent and fuel gas, the projections being provided onthe plate-like member and making contact with the membrane electrodeassembly. One of the gas passages is defined by two adjacentprojections, the plate-like member and the membrane electrode assembly.Further a coefficient of thermal conductivity of the projection issmaller than a coefficient of thermal conductivity of the plate-likemember of the separator.

Furthermore, this invention provides a method of manufacturing aseparator for a fuel cell, the fuel cell having the separator providedwith a plate-like member and a plurality of projections on theplate-like member, the projections making contact with an electrode ofthe fuel cell, wherein a gas flow groove is formed between two adjacentprojections and extends parallel to a surface of the separator. Themethod comprises forming the projections from a material having a lowercoefficient of thermal conductivity than a coefficient of thermalconductivity of the plate-like member; forming a membrane from ahydrophilic electrically-conductive coating applied to the surface ofthe projections and the surface of the plate-like member; and removingthe membrane from a top of the projections which makes contact with theelectrode of the fuel cell by grinding the surface on the top of theprojections or by applying a blasting process after filling the gas flowgroove with a liquid medium or a gel medium.

A separator according to this invention has a plurality of projectionseach making contact with one electrode and has a low coefficient ofthermal conductivity. Thus heat which is generated in proximity to theelectrolytic membrane of the fuel cell remains localized. Consequently,heat generated in proximity to the electrolytic membrane can beeffectively used in order to melt frozen water in the gas diffusionelectrode. Furthermore this melted water can be rapidly discharged fromthe gas diffusion electrode to the gas flow groove.

The details as well as other features and advantages of this inventionare set forth in the remainder of the specification and are shown in theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic partial perspective view of a fuel cell stackaccording to this invention.

FIG. 2 is a schematic perspective view of a separator according to afirst embodiment.

FIG. 3 is a flowchart showing a method of manufacturing a separator.

FIG. 4A is a schematic perspective view of a separator according to asecond embodiment. FIG. 4B is a schematic view of the microstructure ofa rib of the separator in the detail area “IV-B” in FIG. 4A.

FIG. 5 is a graph of the startup characteristics of the fuel cell at lowtemperatures according to the first and second embodiments.

In order to clarify the figures, several members have been colored blackor gray. In the figures, similar elements have similar referencenumerals.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, the structure of a fuel cell stack 1 used in thisembodiment will be described.

A fuel cell stack 1 is provided with a plurality of unit cells 1 aplaced in series. The unit cells 1 a are provided with electrolyticmembrane 2, two electrodes 3 a, 3 b and two separators 4 a, 4 b. Theelectrolytic membrane 2 is formed from a solid polymer. The plate-shapedanode electrode 3 a and cathode electrode 3 b are disposed to sandwichthe electrolytic membrane 2. The electrodes are gas diffusion electrodeswhich are provided with a thin platinum catalytic layer (22 a, 22 b)connected to the electrolytic membrane 2, and a porous gas diffusionlayer (21 a, 21 b) which supports the catalytic layer and is provided onthe outer side of the catalytic layer. The platinum catalytic layer (22a, 22 b) comprises a platinum catalyst supported on carbon carrier. Theporous gas diffusion layer comprises carbon such as a carbon cloth orcarbon paper, and is imparted with water repellent characteristics. Theelectrodes (3 a, 3 b) are provided with a gas diffusion layer (21 a, 21b) in order to allow diffusion of supplied fuel gas or a gaseousoxidizing agent to the electrolytic membrane 2. Since the supplied fuelgas and the gaseous oxidizing agent disperse over the entire surface ofthe electrodes 3 a, 3 b, reactions occur uniformly in the fuel cellstack 1.

The electrolytic membrane 2 and the electrodes 3 a, 3 b comprise amembrane electrode assembly (MEA) 20. The two separators are formedsubstantially in the shape of a plate and comprise an anode separator 4a and a cathode separator 4 b. The two separators sandwich the MEA 20.

A plurality of gas flow grooves 7 a (7 aa) extending in parallel in ahorizontal direction in FIG. 1 are formed on the surface 6 a (6 aa) ofthe anode separator 4 a (4 aa) facing the anode electrode 3 a. In thisdescription, the projection which defines the gas flow groove 7 a (7 aa)is termed a rib 1 la (11 aa). The ribs 11 a of the anode separator 4 a(4 aa) are disposed at equal intervals. Fuel gas required for generatingpower is allowed to flow in the gas flow groove 7 a (7 aa) in order tosupply fuel to the anode electrode 3 a.

On the other hand, a plurality of gas flow grooves 7 b extending inparallel in a vertical direction in FIG. 1 are formed on the surface ofthe cathode separator 4 b facing the cathode electrode 3 b. Theprojection which defines the gas flow groove 7 b is termed a rib 11 b.The ribs 11 b of the cathode separator 4 b are disposed at equalintervals in the same manner as the ribs 11 a. Gaseous oxidizing agentrequired for generating power is allowed to flow in the gas flow groove7 b and is supplied to the cathode electrode 3 b. The gas flow groove 7a (7 aa) supplying fuel gas is formed orthogonal to the gas flow groove7 b supplying gaseous oxidizing agent. However the invention is notlimited in this respect and the passages may be formed parallel to oneanother.

In the anode electrode 3 a using supplied fuel gas, the followingreaction occurs.H₂→2H⁺+2e ⁻  (1)

Electrons pass through an electric wire and a load (not shown) and reachthe cathode electrode 3 b. On the other hand, protons reach the cathodeelectrode 3 b by passing through the electrolytic membrane 2. In thecathode electrode 3 b, electrochemical reactions required for powergeneration are performed as a result of the following reaction betweenoxygen contained in the oxidizing agent and the supplied electrons andprotons.1/2O₂+2H⁺+2e ⁻→H₂O  (2)

The overall temperature of the cell 1 a is increased as a result of thereactions (1) and (2) between the anode electrode 3 a and the cathodeelectrode 3 b.

A plurality of cooling grooves 8 a, 8 b (8 aa) is optionally formed onthe rear face of the separator 4 a, 4 b (4 aa) opposite to the electrodeside in order to cool the cell 1 a. A cooling passage 9 is formed byassembling the cooling grooves 8 a, 8 b (8 aa) in adjacent opposedcells. For example, one cooling passage 9 is formed from the coolinggrooves 8 b, 8 aa respectively placed on the surface of the cathodeseparator 4 b in the cell 1 a and the anode separator 4 aa in theadjacent cell 1 a. The cooling passages 9 are formed in parallel and atequal intervals on the surface of the separator 4 a, 4 b (4 aa). Acooling medium is introduced into the cooling passage 9 in order to coolthe fuel cell stack 1.

Referring to FIG. 2, a separator 4 used in the fuel cell stack 1 will bedescribed. In FIG. 2, the illustration of the cooling grooves isomitted.

In this separator 4, rectangular parallelepiped ribs 11 are disposed inparallel on a plate 10 (or a plate-like member) at equal intervals onthe surface of the plate 10. The ribs 11 are projections projected fromthe plate 10 for forming the plurality of gas passages which allow flowof one of the gaseous oxidizing agent and fuel gas. The top face 23 ofthe rib 11 makes contact with the anode or cathode electrode. Ahydrophilic membrane 14 is formed on the bottom 13 and on both wallfaces 12 of the gas flow groove 7 formed between the ribs 11 disposed inthe above manner.

The ribs 11 are formed from a material which has a lower coefficient ofthermal conductivity than the material constituting the plate 10. Sincethe coefficient of thermal conductivity of the ribs 11 is lower than thematerial constituting the plate 10, heat produced at the electrodes doesnot tend to diffuse through the separator from near to the top of therib 11 which makes contact with the electrode. Consequently it ispossible to use the heat generated at the electrodes to melt water whichhas frozen at low temperatures. As a result, when operating the fuelcell at low temperatures, water which has frozen in the gas diffusionelectrode is melted. As described above, since water is produced in thecathode electrode 3 b, this invention is applied at least to theseparator 4 b connected to cathode electrode 3 b. Furthermore it ispreferred that this invention is applied in the same manner to theseparator 4 a connected to anode electrode 3 a as well. Since it isoften the case that the fuel gas is supplied with the addition of watervapor, it is possible to apply this invention only to the separator 4 aconnected to anode electrode 3 a in order to further improve theprevention of freezing of water resulting from condensation of addedwater vapor.

In the first embodiment, the porosity of the material forming the rib 11is higher than that forming the plate 10. Generally since the separator4 of the fuel cell stack 1 should display electrical conductivity,thermal resistance and acid resistance, a carbon composite or a metalcomponent having an anti-oxidizing coating thereon are widely employedas the material forming the separator 4.

It is preferred that the material forming the rib 11 of the separator 4in this embodiment comprises graphite or a carbon composite which allowrelatively simple control of porosity. Such materials should not displaylarge impediments to conductivity. A preferred method of forming the rib11 comprises a step of molding a composite material from various typesof resins and carbon powder or a step of graphitization comprisingbaking of a non-woven fabric impregnated with resin, due to their costefficiency. Furthermore it is possible to form the rib 11 from graphite,perform a cutting operation to a predetermined size and arrange andattach the resulting rib 11 to the plate 10.

The effective porosity of the material constituting the rib 11 is set to10-80%, preferably to 10-60% and even more preferably to 10-40%. At lessthan a porosity of 10%, it is not possible for the separator to providea sufficiently heat insulating effect. At greater than a porosity of80%, there is the possibility of damage during lamination of theseparator 4 due to extreme weakening of mechanical strength. Thedimensions of the pores may be varied in response to the width and depthof the gas flow groove 7. When the plate 10 is formed from the samematerial as the rib 11, the mechanical strength of the separator itselfis reduced due to high porosity. Furthermore cooling performance duringcooling operations is reduced due to the overall reduction in thecoefficient of thermal conductivity of the separator 4. As a result, itis preferred that the plate 10 is not formed from the same material asthe rib 11.

A hydrophilic membrane 14 is formed by coating a slurry or a coatingcontaining a hydrophilic material onto only the bottom 13 and both wallfaces 12 of the gas flow groove 7. In this embodiment, heat generated inproximity to the electrolytic membrane 2 is consumed by melting thefrozen water in the gas diffusion electrode (electrodes 3 a, 3 b). Inorder to improve the initial power generation efficiency at lowtemperatures, it is preferred that residual frozen water in theelectrode (3 a, 3 b) is discharged rapidly from the electrode (3 a, 3 b)to the gas passage. For this reason, the hydrophilic membrane 14 isformed only on the bottom 13 and both wall faces 12 of the gas flowgroove 7. Since the hydrophilic membrane 14 facilitates discharge ofmelted water from the inside of the gas diffusion electrode into the gasflow groove 7, the water discharge performance of the electrodes (3 a, 3b) is improved. In particular, when the gas diffusion layer (21 a, 21 b)of the electrodes (3 a, 3 b) has a water-repellent effect, the waterdischarge performance of the electrodes (3 a, 3 b) is further improved.Since the top face 23 of the rib 11 contacting to the MEA 20 is notprovided with a hydrophilic membrane 14, it is possible to avoidreductions in porosity in the section connecting the separator 4 withthe electrode (3 a, 3 b).

Referring to the flowchart in FIG. 3, a method of forming the separator4 will be described below.

Firstly in a step S1, the separator 4 is formed. The separator 4 may beformed using a known method. For example, the separator 4 may be formedby cutting a gas flow groove 7 with a mechanical process into agraphitized plate 10 or by compression molding, injecting molding orextrusion molding of a composite material comprising carbon powder andvarious types of resin.

Then in a step S2, a coating containing a material with hydrophilic andelectrical-conductive properties is coated onto the entire surface ofthe separator 4. The method of coating the surface of the separator 4may include various known methods such as a spray method, a castingmethod or a dip coating method. A gentle abrasive process may bepre-applied to the surface of the separator 4 in order to ensuresufficient adhesion of the coating.

Then the routine proceeds to a step S3 wherein the coating is dried. Thedrying process for the coating can be performed using a known methodsuch as natural drying, hot air drying or drying methods using varioustypes of electromagnetic waves. In this manner, the hydrophilic membrane14 is formed on the entire surface of the separator 4.

After forming the hydrophilic membrane 14 on the surface of theseparator 4, the processes in the steps S4, S5 are performed to removethe hydrophilic membrane from surfaces other than the bottom 13 and bothwall faces 12.

In the step S4, the gas flow groove 7 of the separator 4 is filled witha gel or a liquid in order to perform a masking operation on the bottom13 and both wall faces 12 of the gas flow groove 7. Before gas flowgroove 7 of the separator 4 is filled with gel or liquid, the gasinlet/outlet of the separator 4 is pre-sealed. In this manner, noleakage occurs even when the gas flow passage 7 is filled with gel orliquid. It is preferred that a jig is used which is adapted to the gasinlet/outlet and the shape of the separator 4. Water is preferred as aliquid for filling the groove 7 due to cost considerations. Howeveranother liquid medium or a liquid with high viscosity may be selected,in consideration of requirements in subsequent processes or interactionwith the hydrophilic coating.

Then in a step S5, the hydrophilic coating on surfaces other than thebottom 13 and both wall faces 12 of the gas flow groove 7 is removed. Itshould be noted that the hydrophilic coating on the top face 23 of therib 11 is removed herein. A blasting method may be used in order toremove the hydrophilic coating. The blasting process may be either aknown air blasting process or a shot blasting process. In the blastingprocess, metallic oxidized metal, resin or glass particles may be usedwith the radius and hardness thereof being selected as required. Thus,after the liquid or gel fills the gas flow groove 7, the hydrophilicmembrane 14 is removed from areas not covered by the gel or the liquidby application of the blast process.

Finally the routine proceeds to a step S6, the gel or liquid filling thegas flow groove 7 is removed and the separator 4 is completed.

There is a grinding method by contact with the surface of the separator4 in order to leave the hydrophilic membrane 14 only on the bottom 13and both wall faces 13 of the gas flow groove 7 while removing themembrane 14 from other areas (corresponding to the steps S4-S5). Of thevarious known methods of grinding, there is a method of applying aroller-shaped grinding member reciprocally over the separator 4 on whichthe hydrophilic membrane 14 is formed. In this manner, it is possible toperform the surface processing without resulting in peeling of thehydrophilic membrane 14 on the bottom 13 or both wall faces 12 of thegas flow groove 7.

Referring to FIG. 4A, a separator 4 according to a second embodimentwill be described. In FIG. 4A, the illustration of the cooling groovesis omitted. In the second embodiment, the rib 11 shown in FIG. 2displays anisotropy with respect to the coefficient of thermalconductivity.

In order to prevent unintended diffusion of heat generated in proximityto the electrolytic membrane 2, the coefficient of thermal conductivityof the rib 11 (projection of the separator 4) with respect to the depth“A” of the gas flow groove 7 is smaller than the coefficient of thermalconductivity in the direction “B” which is orthogonal to the depth “A”.Namely, the coefficient of thermal conductivity in a direction of adepth of the projection (rib 11) of the separator 4 is smaller than thecoefficient of thermal conductivity in a direction orthogonal to thedepth. The material constituting the rib 11 has anisotropy with respectto the coefficient of thermal conductivity in addition to severalcharacteristics (including high electrical conductivity) required as amaterial for the separator.

The material for the rib 11 comprises plate-like graphite grains (orgraphite flakes) obtained from application of a roller for pulverizing ablock of graphite, natural graphite flakes, or a composite material fromresin and anisotropic graphite flakes.

Graphite grains in plate form (or graphite flakes) have a highcoefficient of thermal conductivity along the direction of the plate anda small coefficient of thermal conductivity along the directionorthogonal to the plate. For the purpose of forming a rib 11,anisotropic graphite flakes or a composite material comprising graphiteflakes and resin may be processed by injection molding, compressionmolding or extrusion molding under a unidirectional pressure operation.In this manner, a rib 11, which displays anisotropy with respect to thecoefficient of thermal conductivity, can be manufactured such that thegraphite grains in plate form (or graphite flakes) in the rib 11 areoriented in a vertical direction “B” which is orthogonal to the depth“A”. FIG. 4B shows schematically the microstructure of the rib 11 formedfrom a composite material comprising graphite grains 31 in plate formand resin, wherein the graphite grains 31 in plate form are oriented inthe rib 11. The graphite grains in plate form (or graphite flakes) maybe orientated in a vertical direction “B” by naturally piling up thegraphite grains under gravity in resin melted during molding. Thus a rib11 manufactured in this manner displays an anisotropic coefficient ofthermal conductivity.

In the rib 11, it is preferred that the thermal conductivity withrespect to depth “A” is {fraction (1/45)} to ½ of the thermalconductivity in the vertical direction “B”. If the ratio of the thermalconductivity with respect to depth “A” to the thermal conductivity in avertical direction “B” is greater than ½, heat insulation performance ispoor and heat generated by the MEA 20 diffuses. If the ratio is lessthan {fraction (1/45)}, the diffusion of heat with respect to depth “A”is extremely poor and as a result, there is the possibility of anadverse effect on the cooling performance of the cooling medium flowingin the cooling passages 9 during operation of the fuel cell stack 1.

After forming the separator 4 including the rib 11 of the above type, ahydrophilic membrane 14 is formed on the bottom 13 and both wall faces12 of the gas flow groove 7 in the same manner as the first embodiment.

Use of a material displaying anisotropy with respect to the thermalconductivity for the rib 11 makes it possible to prevent diffusion ofheat generated by the electrolytic membrane 2 with respect to the depth“A” of the separator 4 without adverse effects on the electricalconductive properties. Consequently the rib 11 according to the secondembodiment enables rapid melting of frozen water at low temperatures.

The invention is illustrated in more detail by the following examples,which are illustrative of specific modes of practicing the invention andare not intended as limiting the scope of the invention.

EXAMPLES Example I

EXAMPLE I corresponds to the first embodiment of the invention. Theconstituent material of the rib 11 is a composite material “M1” of resinand artificial graphite powder displaying low anisotropy but highporosity of 15%. The rib 11 displays a coefficient of thermalconductivity of 4.6 W/mK in a direction “B” orthogonal to the depth “A”and a coefficient of thermal conductivity of 5.2 W/mK with respect tothe depth “A”. The resistivity of the rib 11 is 15.8 Ω·cm in thevertical direction “B”.

Example II

EXAMPLE II corresponds to the second embodiment of the invention. Theconstituent material of the rib 11 is a composite material “M2” of resinand expanded graphite powder in flake form displaying high anisotropyand low porosity of 3.2%. The rib 11 displays a coefficient of thermalconductivity of 125 W/mK in a direction “B” orthogonal to the depth “A”and a coefficient of thermal conductivity of 3.5 W/mK with respect tothe depth “A”. The resistivity of the rib 11 is 15.8 Ω·cm in thevertical direction “B”.

Comparative Example I

The constituent material of the rib 11 is a composite material “M3” ofresin and artificial graphite powder displaying low anisotropy and lowporosity of 4.2%. The rib 11 displays a coefficient of thermalconductivity of 5. 8 W/mK both in a direction of the depth “A” and in adirection “B” orthogonal to the depth “A”. The resistivity in adirection “B” orthogonal to the depth “A” is 16 Ω·cm.

In EXAMPLE I, II and COMPARATIVE EXAMPLE I, the porosity was measuredusing a mercury porosimeter. The resistivity was measured using afour-probe method.

In EXAMPLE I, II and COMPARATIVE EXAMPLE I, the material of the plate 10of the separator 4 is the composite material “M3”. In EXAMPLE I, II,after the composite material “M3” is poured into a first section of themold for shaping the separator 4 (the first section corresponds to theplate 10), the composite material “M1” or “M2” is poured into a secondsection of the mold for shaping the separator 4 (the second sectioncorresponds to the rib 11). Thereupon a compressive molding operation isperformed. In COMPARATIVE EXAMPLE I, the compression molding operationis performed after pouring the composite material “M3” into the entireregion of the mold for shaping the separator 4. The size of theseparator 4 is 100×100×3 mm with gas flow grooves 7 of a width of 2 mm,and a depth of 1.5 mm. The gas flow grooves 7 are disposed at intervalsof 2 mm.

In EXAMPLE I, II and COMPARATIVE EXAMPLE I, the coating for forming thehydrophilic membrane 14 is a coating which includes carbon black, liquidphenol and polyvinyl alcohol all dissolved in methanol. The coating isperformed by an air spray process and dried in order to form thehydrophilic membrane 14. The coating is dried at 50° C. for one hourusing heated air and thereafter is allowed to dry for 12 hours at 70° C.After the coating is dried, the inlet/outlet of the gas flow groove 7 isclosed and the gas flow groove 7 is filled with water. Thereafter ablasting process is applied to the top surface 23 of the separator 4with an air pressure of 2.5 kg/cm² using aluminum particles with anaverage diameter of 350 μm.

In EXAMPLE I, II and COMPARATIVE EXAMPLE I, the gas diffusion layer inthe electrodes (3 a, 3 b) is a carbon cloth with a thickness of 300 μmhaving a water repellent function.

Referring to FIG. 5, the output voltage characteristics during startupafter freezing of a fuel cell provided with a separator 4 will bedescribed with reference to EXAMPLE I, II and COMPARATIVE EXAMPLE I. Theexperimental method of examining the output voltage characteristics ofthe fuel cell is described below.

Hydrogen and oxygen (not containing water vapor) are used as a fuel andan oxidizing agent for the fuel cell stack 1, respectively. Aconstant-current electronic load device is connected to the fuel cellstack 1 and the generated current is controlled to a fixed value. Athermo-couple for measuring temperature at a specific position ismounted about a specific cell 1 a of the fuel cell stack 1.

Firstly at low temperatures, the fuel cell stack 1 generates power for aperiod of five seconds with a current density of 0.5 A/cm². In thismanner, moisture is absorbed by the gas diffusion electrode (electrodes3 a, 3 b). Thereafter the ambient temperature about the fuel cell is setat −5° C. The fuel cell stack 1 is left to cool until the detectedinternal temperature of the cell 1 a reaches −2° C. Consequently, waterabsorbed in the gas diffusion electrode becomes frozen. The fuel cellstack 1 is then re-started in this state. The temporal variation in theoutput voltage during startup is recorded as the output voltagecharacteristics. The graph shown in FIG. 5 shows temporal variation inthe cell voltage at a current density of 0.5 A/cm².

Referring to FIG. 5, a separator 4 according to COMPARATIVE EXAMPLE Iusing a conventional technique will be compared with a separator 4according to EXAMPLE I or II. A fuel cell stack 1 using a separator 4according to EXAMPLE I or II can maintain a high cell voltage for a longperiod of time in comparison with a fuel cell stack 1 using a separator4 according to COMPARATIVE EXAMPLE I. In comparison with the fuel cellstack 1 using a separator 4 according to COMPARATIVE EXAMPLE I, the fuelcell stack 1 using a separator 4 according to EXAMPLE I or II generatesa high cell voltage immediately after startup.

As a consequence, a fuel cell using a separator according to thisinvention displays excellent startup characteristics even when the fuelcell has cooled to a low temperature of less than zero ° C.

The entire contents of Japanese Patent Application P2001-299500 (filedSep. 28, 2001) are incorporated herein by reference.

Although the invention has been described above by reference to certainembodiments of the invention, the invention is not limited to theembodiments described above. Modifications and variations of theembodiments described above will occur to those skilled in the art, inlight of the above teachings. The scope of the invention is defined withreference to the following claims.

1. A separator for a fuel cell, the fuel cell having two separators anda membrane electrode assembly being sandwiched by the two separators,the membrane electrode assembly being provided with an anode electrode,a cathode electrode and an electrolytic membrane sandwiched by the twoelectrodes, each of the two electrodes having a diffusion layer allowingdiffusion of one of a gaseous oxidizing agent and fuel gas; theseparator comprising: a plate-like member; and a plurality ofprojections for forming a plurality of gas passages which allow flow ofone of the gaseous oxidizing agent and fuel gas, the projections beingprovided on the plate-like member and making contact with the membraneelectrode assembly, one of the gas passages being defined by twoadjacent projections, the plate-like member and the membrane electrodeassembly; wherein a coefficient of thermal conductivity of theprojection is smaller than a coefficient of thermal conductivity of theplate-like member of the separator; and wherein in the projections, thecoefficient of thermal conductivity in a direction of a depth of theprojection is smaller than the coefficient of thermal conductivity in adirection orthogonal to the depth.
 2. The separator for a fuel cell asdefined in claim 1, wherein a coefficient of thermal conductivity of amaterial forming the projection is smaller than a coefficient of thermalconductivity of a material forming the plate-like member of theseparator.
 3. The separator for a fuel cell as defined in claim 2,wherein the porosity of the material forming the projection is greaterthan the porosity of the material forming the plate-like member of theseparator.
 4. The separator for a fuel cell as defined in claim 3,wherein the material forming the projection has a porosity between 10%to 80%.
 5. The separator for a fuel cell as defined in claim 1, whereinthe coefficient of thermal conductivity in a direction of the depth ofthe projection is {fraction (1/45)} to ½ of the coefficient of thermalconductivity in a direction orthogonal to the depth.
 6. The separatorfor a fuel cell as defined in claim 1, wherein the material forming theprojection contains plate-like graphite grains oriented in a directionorthogonal to a depth of the projection.
 7. A separator for a fuel cell,the fuel cell having two separators and a membrane electrode assemblybeing sandwiched by the two separators, the membrane electrode assemblybeing provided with an anode electrode, a cathode electrode and anelectrolytic membrane sandwiched by the two electrodes, each of the twoelectrodes having a diffusion layer allowing diffusion of one of agaseous oxidizing agent and fuel gas; the separator comprising: aplate-like member; and a plurality of projections for forming aplurality of gas passages which allow flow of one of the gaseousoxidizing agent and fuel gas, the projections being provided on theplate-like member and making contact with the membrane electrodeassembly, one of the gas passages being defined by two adjacentprojections, the plate-like member and the membrane electrode assembly;wherein a coefficient of thermal conductivity of the projection issmaller than a coefficient of thermal conductivity of the plate-likemember of the separator; and wherein only bottom face and both wallfaces of the gas passage are covered by a membrane displayinghydrophilic characteristics, wherein the bottom face and the wall facesis included in a surface of the separator.
 8. A method of manufacturinga separator for a fuel cell, the fuel cell having the separator providedwith a plate-like member and a plurality of projections on theplate-like member, the projections making contact with an electrode ofthe fuel cell, wherein a gas flow groove is formed between two adjacentprojections and extends parallel to a surface of the separator; themethod of manufacture comprising: forming the projections from amaterial having a lower coefficient of thermal conductivity than acoefficient of thermal conductivity of the plate-like member; forming amembrane from a hydrophilic electrically-conductive coating applied tothe surface of the projections and the surface of the plate-like member;and removing the membrane from a top of the projections which makescontact with the electrode of the fuel cell by grinding the surface onthe top of the projections or by applying a blasting process afterfilling the gas flow groove with a liquid medium or a gel medium.
 9. Afuel cell comprising: a membrane electrode assembly comprising anelectrolytic membrane, and an anode electrode and a cathode electrodesandwiching the electrolytic membrane, the anode electrode and thecathode electrode formed from a porous material allowing diffusion ofgas; two separators for supporting and sandwiching the membraneelectrode assembly, each of the separators having a plate-like memberand a plurality of projections projected from the plate-like member fordefining a plurality of gas flow grooves on the surface of theplate-like member; the gas flow grooves allowing flow of one of a fuelgas and a gaseous oxidizing agent; wherein in the separators, acoefficient of thermal conductivity of the projections is smaller than acoefficient of thermal conductivity of the plate-like member; andwherein the projections, the coefficient of thermal conductivity in adirection of a depth of the projection is smaller than the coefficientof thermal conductivity in a direction orthogonal to the depth.